1. Field of the Invention
The present invention relates to a method of measuring the representative shear friction factor at the interface between a workpiece material and forming dies during bulk forming processes and, more particularly, to a method of more easily, effectively, and accurately measuring such representative shear friction factor through the backward extrusion process.
2. Description of the Prior Art
As well known to those skilled in the art, bulk forming processes have been typically and widely used for producing a variety of mechanical parts having different structures and operational functions. Particularly, such a bulk forming process can produce intermediate products having a shape and size similar to those of the desired final product, and so the bulk forming process reduces the number of operations for production and leads to material saving. In addition, bulk forming processes lead to good mechanical properties of the final products due to work hardening of the material during forming. In order to more effectively, or even optimally design such bulk forming processes, numerical analyses are frequently used while designing the process.
In order to use the numerical analysis in the design of a target bulk forming process effectively, it is necessary to accurately describe a variety of numerical parameters. Of such parameters to be estimated, the friction condition at the interface between the workpiece material and forming dies is very important since it directly influences both flow of the workpiece material and forming load during the target bulk forming process. In the end it will affect the success or failure of the process.
In order to express the friction condition quantitatively in bulk metal forming, a constant shear friction model is typically used in numerical analyses as expressed by the following equation (1).
Frictional stress=mfxc3x97shear yield stressxe2x80x83xe2x80x83(1) 
wherein,
mf: shear friction factor
In the above equation (1), the frictional stress is in proportion to the shear yield stress of the material, with the shear friction factor xe2x80x9cmfxe2x80x9d determining the ratio of the proportional relationship. In general, this value is locally varying, depending on the surface quality, the type of lubrication, and the deformation condition at the interface.
Therefore, it is necessary to accurately measure the representative shear friction factor xe2x80x9cmfxe2x80x9d as a single value in order to properly estimate the friction conditions in the target bulk forming process for simplicity and convenience for the process simulation.
A ring compression test has been most typically and widely used for measuring such shear friction factors.
In such ring compression test, variation in the inner diameter of an annular test specimen is measured during the compression of the test specimen to estimate the friction condition. This ring compression test is advantageous in that it is simple in its testing process. However, because this ring compression test is so exceedingly simple in its testing process, it may not be suitable for estimating the friction conditions in more complex bulk forming processes. In addition, the free surface generated during the ring compression test is quite small when compared with those of typical bulk forming processes. Another disadvantage of the ring compression test resides in that it is necessary to use nonlinear calibration curves to determine the desired shear friction factors. According to this method, since the shear friction factor is dependent on the deformation history, it is not easy to determine the representative constant shear friction factor.
Several methods of measuring shear friction factors that overcome the above-mentioned problems experienced in ring compression tests have been recently proposed and implemented. Of those recently proposed methods, various methods based on the backward extrusion process that is capable of generating a large amount of free surfaces have been preferred.
One such a measuring method using the backward extrusion process predicts the shear friction factor by measuring the forming load during the process depending on the friction condition. However, this method of measuring the shear friction factor by measuring the difference in forming loads can be problematic in that the forming load has a direct connection with flow stress of the workpiece material, and thus it is necessary to accurately know flow stress of the material. Such a forming load can be considered only as an indirect measure of the friction condition.
Another method of measuring the shear friction factor based on a simultaneous forward and backward extrusion process has been proposed and used. This method is designed to determine the shear friction factor by measuring the ratio of material flow in the forward and backward ends in accordance with the friction condition. However, this measuring method is problematic in that it is insensitive at high levels of friction.
Accordingly, the present invention has been made with the aforementioned problems in mind. The objective of the present invention is to provide a method of measuring the representative constant shear friction factor at the interface between the workpiece material and forming dies during bulk forming processes through a backward extrusion process as a single value. This method can easily and effectively measure the constant shear friction factors for a variety of friction conditions and is suitable for estimating the friction condition of the complex bulk forming process.
In order to accomplish the above objective, the present invention uses a backward extrusion process, comprised of the following steps: positioning the workpiece material at a predetermined groove location within the forming die and pressurizing the workpiece material to form an extruded product with an apex formed on the backward extruded end; measuring the perpendicular distance xe2x80x9cdxe2x80x9d from the external side surface of the extruded product to the apex; and calculating the shear friction factor xe2x80x9cmfxe2x80x9d using the measured perpendicular distance xe2x80x9cdxe2x80x9d.
In the above measuring method, the workpiece material has a diameter equal to the average of the outer diameter of the punch and the inner diameter of the forming die, and is positioned such that its central axis is aligned with the central axes of both the punch and the forming die.
In addition, the representative shear friction factor xe2x80x9cmfxe2x80x9d, which has a range of 0.0xcx9c1.0, is calculated from the fact that the perpendicular distance xe2x80x9cdxe2x80x9d has a linear relationship with the shear friction factor xe2x80x9cmfxe2x80x9d.
Typically, numerical analyses are attractive for the design of bulk forming processes since they improve design efficiency, save both production time and material resources, and improve the quality of final products of the process. It is necessary to describe the friction condition properly and accurately in order to improve the reliability of numerical analyses of bulk forming processes. When the representative shear friction factor can be properly measured for the whole process as described above, it will be possible to select proper lubricants for target bulk forming processes.
Proper lubrication in bulk forming processes can lead to reduction of forming loads, extension of die life, and reduction of energy consumption. Also, such an efficient and accurate measurement of friction conditions can aid the development of environment friendly lubricants for general use in bulk forming processes.